It is entirely possible to obtain a visitors visa for a relative and an accompanying caregiver to obtain medical treatment in the US (as a matter of fact, I have assisted someone by writing the necessary doctors letters to the consulate in the recent past).
You have to show that:
- a genuine medical condition exists (through a letter/letters from the treating physicians in your sisters home country)
- options for treatment in your home country are limited/unaccessible
- options for treatment in the US exist (through a letter from a physician in the respective specialty in the US who is willing to accept the referral. This letter should also spell out that based on the referral documentation from the home country he/she anticipates a certain length of treatment, what procedures/treatments are anticipated etc.)
- you have the financial means to get your relative access to this treatment in the US without a risk of the US taxpayer having to pay for it (e.g. by 'dumping' your family member in a local ER which is obliged to treat anyone. You and other family members might have to file an 'affidavit of support' with USCIS to demonstrate this ability to pay.)
- your relative has an unabandoned place of living and ties to your home country to go back to.
A good immigration lawyer would be able to assist you in the process. In cases like this it is possible to get visa interviews for the relative on an emergency basis.
This will WON'T provide your sister with an avenue to immigration and I would strongly discourage you from using this kind of compassionate issuance of visitors visas as a backdoor to immigration. But if this is about getting her medical treatment, there are options.
Good Luck, and the best for your sister.